In a medium skillet over medium-high heat, heat oil until shimmering. Add mushrooms, onions, and 1/4 teaspoon salt and stir to combine.
Reduce heat to medium, cover, and cook until liquid is released from mushrooms, about 5 minutes. Remove cover and continue to cook until liquid has evaporated and mushrooms and onions are softened and browned, about 5 minutes longer.
Stir in balsamic vinegar and stir until mushrooms and onions are coated evenly and the vinegar is heated through, about 2 minutes.
White button mushrooms: I call for the classic, and what I think is the mildest and most family-friendly of the umami-boosting mushrooms, white button. See the FAQ section for more options.
Onion: You'll want half of a medium onion, thinly sliced, which is equal to about 1 cup loosely filled. For slightly milder onion flavor, try shallots.
Yield: This Balsamic Mushrooms and Onions recipe makes about 2 cups, enough for four 1/2-cup servings.
Storage: Store leftovers covered in the refrigerator for up to 4 days.